Method for determining the phosphorus content of an aqueous sample, in particular drainage-water sample, wherein the sample is subjected to thermal-oxidative decomposition and the orthophosphate content of the decomposed sample is determined photometrically in an aqueous analysis solution, such that the thermal decomposition is carried out in a single step by catalyzer-free burning of the sample as batch-decomposition in a combustion oven, the resulting combustion gas is transferred out of the combustion oven in a carrier-gas stream and the combustion/carrier gas stream is cooled to obtain the aqueous analysis solution as a condensate therefrom.